Cobalt and nickel are frequently added to gold plating compositions to increase the hardness of the electrodeposited metal; when composition and plating conditions are closely controlled and contamination is minimized, bright deposits are readily obtained over a satisfactory range of current densities. Unfortunately, contamination with various metals frequently occurs and the stability of the bath is affected significantly; moreover, the current efficiency of the bath may be reduced rapidly.
Phosphonic acid chelating agents have long been proposed as components of gold and other metal plating baths to chelate contaminants such as copper and lead. Moreover, it has been recognized that iron contamination can be minimized in baths using a phosphate electrolyte since the phosphate will react with the iron to produce a precipitate.
Exemplary of the baths containing phosphonic acid chelating agents are U.S. Pat. No. 3,770,596 granted Nov. 6, 1973 to Bick et al; U.S. Pat. No. 3,706,634 granted Dec. 19, 1972 to Kowalski; and U.S. Pat. No. 3,904,493 granted Sept. 9, 1975 to Losi et al. Bick et al U.S. Pat. No. 3,856,638 granted Dec. 24, 1974 is of interest in proposing that the nickel and cobalt be reacted with a phosphonic acid compound and aminoguanidine.
The use of various other organic compounds as chelating agents, brighteners, levelling agents and the like has long been known, and the patent and technical literature contain many compounds and combinations of compounds proposed to offer various advantages. Moreover, buffering agents such as citric acid, boric acid, malic acid and the like have also been proposed to control the pH of the bath.
Despite the various known combinations of compounds that might be added to a plating bath for electrodepositing a hard gold alloy, there has remained a continuing problem to provide a stable bath which would operate at high current efficiency in a wide range of current densities and without requiring a change in bath makeup. Moreover, the industry desires all purpose baths which can be used for rack, barrel and high speed plating applications with reasonable efficiency.
It is an object of the present invention to provide a novel gold plating bath which contains cobalt and/or nickel as a hardener, which is stable and efficient over a wide range of current density and which may be used for rack, barrel and high speed applications.
It is also an object to provide such a bath which may be formulated readily and relatively economically and which is highly effective in resisting the effects of copper, lead and iron contamination.
Another object is to provide a novel and highly efficient method for electrodepositing hard, bright gold alloy deposits over a wide range of current densities and in various types of plating applications.